This invention relates to color cathode ray tubes (CRTs) having vertically striped phosphor screens and slotted aperture masks, and more particularly relates to such CRTs having smooth top and bottom screen edges.
Color CRTs for color television produce an image display on a cathodoluminescent screen composed of a repetitive array of red, blue and green phosphor elements, by scanning the array with three electron beams from an electron gun in the neck of the CRT, one beam for each of the primary (red, blue and green) colors. The beams emanate from separate gun apertures, converge as they approach the screen, pass through an aperture mask positioned a short distance (designated "Q") behind the screen, and then diverge slightly to land on the appropriate phosphor element. At a comfortable viewing distance, the human eye cannot resolve the individual red, blue and green elements in the screen, but rather integrates these primary colors to perceive additive colors produced by the primary colors.
Early CRTs for color television had screens composed of arrays of phosphor dots, but dot screens have been largely replaced by screens composed of arrays of vertically oriented phosphor stripes. As is known, such screens are primarily advantageous in alleviating the requirement for accurate registration between the mask and screen in the vertical direction.
The masks for these striped screens are composed of vertically oriented columns of slot-shaped apertures separated from one another by so-called "tie bars" of mask material, which "tie" the mask together to provide needed mechanical strength. The slots in adjacent columns are usually staggered in some manner in order to avoid moire, an annoying beat pattern on the screen caused by slight, unintentional misregistration between the mask, screen and raster scan patterns.
Unfortunately, this staggering of the slots, together with the convex curvature conventionally designed into the top and bottom edges of the mask array, results in aperture columns ending randomly in partial slots of varying length or in tie bar zones, and leaves the appearance of a ragged screen edge.
In U.S. Pat. No. 4,300,070, this problem is addressed by changing the length of the end portions of the aperture columns in a border region of the mask. Specifically, in all columns in which terminal slots would be less than one-half the length of a full slot, the last full slot is lengthened to end at the edge of the border. However, this approach tends to require rather complex mask designs, and also tends to result in changes in the brightness of the screen display image in this border region.
Accordingly, it is an object of this invention to provide in a color CRT with a striped screen and a slotted aperture mask, an aperture mask which enables smooth top and bottom screen edges.
It is a further object of the invention to provide such a mask which is relatively simple in design and does not require non-uniform changes in the border regions of the mask array.